This invention relates to methods of producing bearing surfaces in the opening or xe2x80x9ceyexe2x80x9d of a connecting rod such as used in automobile engines.
Conventional connecting rods, in particular those for internal combustion engines, are designed as so called xe2x80x9ccutxe2x80x9d connecting rods or xe2x80x9ccrackedxe2x80x9d connecting rods. In this case, it is the large connecting rod eye which surrounds the crankshaft that is cut or cracked. The small connecting-rod eye generally does not need to be opened since it is connected to the piston by a straight pin which can be passed through the eye.
Connecting rod eyes are customarily fitted with one of several different bearing shells i.e., sliding bearings, depending on the load to be applied to the bearing. Backing shell materials used in particular as bearing shells are generally made of C 10 steel according to DIN Standard 17210 or SAE Standard 1010. Depending on its constructional shape and use, a bearing shell may be cold-hardened. The actual sliding layer of the sliding bearing which is applied to the backing shell material may be, for example white metal, leaded bronze, light metal, a sputter layer or the like, which is selected according to the anticipated load on the bearing. The bearing shell may be designed as a ternary, binary or solid bearing shell or in other ways. In order for the bearing shell to achieve a perfect fit after assembly, it is mounted with prestress.
The bearing shell not only constitutes a material cost factor, it also entails production expenditure and constitutes a source of error. Thus, for example, the insertion of a bearing shell or a half bearing shell may be overlooked during assembly, resulting in considerable damage to the engine.
Accordingly, it is an object of the present invention to provide a method for producing a connecting rod eye which overcomes disadvantages of the prior art.
Another object of the invention is to provide a method for producing a connecting rod bearing with reduced assembly costs.
According to the invention, a bearing shell is no longer required in a connecting rod eye, and particularly a large connecting rod eye, and possibly even both connecting rod eyes. Instead, a bearing layer is applied directly to the surface of a connecting rod eye by thermal spraying. Plasma spraying is used preferably as thermal spraying and the connecting rods are preferably parts of an internal combustion engine.
As the bearing layer a metal, and preferably a metal alloy, is sprayed on. Moreover, a mixture of different metals, preferably metal alloys, is used. In one embodiment the mixture may be, for example, a mixture of aluminum and tin, in which case the mixture is obtained particularly advantageously by mixing the individual components in powder form. The classification of the metal powder depends, inter alia, on the individual spraying parameters and can easily be determined by the person skilled in the art in a series of tests. As bearing materials, bronzes are advantageously used, in particular aluminum bronze or aluminum/tin or even copper bronze, as well as metal/soft material layers and/or metal/solid lubricant layers. Soft metals, such as lead, are used, for example, as soft materials, which are dispersed in a harder metal such as, for example, alloyed aluminum such as aluminum/copper/magnesium/chromium. However, other materials such as, for example, fluoropolymers, e.g. polytetrafluoroethylene, can also be used as soft materials. Examples of suitable solid lubricants are compounds such as molybdenum disulfide, boron nitrite and graphite.
The bearing material is sprayed on particularly advantageously to produce an oversize layer which is subsequently reworked mechanically. Honing is used, for example, for reworking. In particular, fine spindling is suitable. In the case of honing, preferably about 20 xcexcm to 300 xcexcm, and in particular about 50 xcexcm to 200 xcexcm, of the bearing material is removed. In the case of fine spindling, the oversize is advantageously 50 xcexcm to 1000 xcexcm, and in particular 100 xcexcm to 500 xcexcm.
The finished bearing layer, after removal of the oversize, is preferably about 150 xcexcm to 800 xcexcm thick, and in particular about 200 xcexcm to 500 xcexcm thick. Such a layer is far thinner than conventional bearing shells, which have a thickness of about 2.5 mm. This means that either more material around the edge of the connecting rod remains, thereby permitting a higher loading, or that connecting rods of lower weight can be produced.
According to the invention, the connecting rod eye which is coated with the bearing material by thermal spraying is advantageously separated into two parts only after the bearing material has been applied. This procedure is used particularly when the connecting rod eye is to be broken open. In this process, the inner surface of the connecting rod eye is scored with a notch at each of the desired breaking points. The notches are preferably made by a laser, in which case a beam from a solid state laser, for example, is introduced at about 45xc2x0 into the connecting rod eye. The laser power used is in particular, about 5 to 10 kW. The notch preferably has a width of about 0.3 mm to about 0.8 mm and a depth of about 0.2 mm to about 0.7 mm. The breaking point is usually located approximately centrally in the connecting rod eye. Alternatively, the notch can also be made by a broaching tool, for example by impact.
The procedure followed when notching and breaking the connecting rod eye is preferably such that the connecting rod eye is first notched, then the bearing material is applied by thermal spraying, and subsequently the breaking is carried out. By this procedure, the gap which is otherwise present at the breaking point or between the individual bearing shells is reduced or eliminated. Under a high engine load, such a gap causes the oil film to tear away. By notching prior to coating of the bearing material, improved lubricating properties are achieved. Under some circumstances, breaking can also take place prior to the coating of the bearing material, which is then broken again, preferably without notching.
If the connecting rod eye is opened by being cut open, this preferably takes place before the bearing material is applied by thermal spraying. In this procedure, the connecting rod eye is cut open, and the separation surfaces of the remaining connecting rod and the connecting rod bearing cover are smoothed individually by broaching. The parts are subsequently fitted together again, provided with bores and threads and screwed together. In this case as well a notch is advantageously made at the separation point in the connecting rod eye which has been screwed together again, preferably prior to coating with the bearing material. Subsequently, the connecting rod eye is broken open again to separate the bearing layer. If the breaking takes place too unevenly in this case, the bearing layer itself has to be notched.
The honing or spindling treatment of the bearing layer to remove excess material as described above takes place particularly advantageously only after the connecting rod eye has been opened. As a result, any burr which may occur at the breaking edge when the bearing layer is broken is removed at the same time as the excess material is removed.
An oil duct can also be provided in the eye of a connecting rod produced according to the invention. The oil duct is advantageously bored into the connecting rod eye only after the bearing layer has been applied and, in particular, after the bearing layer has been reworked. In this case as well, the oil duct can be bored through the connecting rod itself so as to extend to the opposite connecting rod eye.
According to the invention, the large flat side surfaces of the connecting rod are advantageously reworked only after the bearing layer has been applied. This reworking preferably takes place by grinding the flat surfaces.
In the method according to the invention, the bearing surface of the connecting rod eye is preferably roughened prior to applying the coating. The roughening is preferably accomplished by sandblasting. However, blasting using a liquid at high pressure is also possible. In this case, the material of the eye, in particular a C 70 steel, is advantageously roughened to an average peak-to-valley height Ra of about 4 xcexcm to about 30 xcexcm, in particular about 8 xcexcm to about 12 xcexcm. With such average peak-to-valley heights, particularly good adherence of the bearing material on the material of the connecting rod eye is achieved.
For particularly advantageous production of the bearing layer in the connecting rod eye, at least one of the flat side surfaces of the connecting rod eye is covered with a template which has an opening in the region of the connecting rod eye. The template opening should be approximately the same size as the connecting rod eye, so that, on the one hand, the template does not interfere with the coating operation and, on the other hand, coating of the flat side surface in the region of the connecting rod eye is largely avoided. If only one flat surface is covered with the template, the other flat surface advantageously lies on a pallet which, like the template, has an opening in the region of the connecting rod eye.
According to the invention, the connecting rod eyes of several connecting rods are advantageously coated in one operation. For this purpose, a plurality of connecting rods, advantageously 2 to 10, and in particular 4 to 8, are positioned one above another in such a way that the connecting rod eyes to be coated form a cylinder. For this purpose, a holder may be provided on a special pallet so that the holder centers the connecting rods placed in the holder. The connecting rods, which are coated virtually simultaneously in this method, are advantageously kept together as a group in order subsequently to be installed together in the same internal combustion engine. All of the connecting rods of an internal combustion engine are thus advantageously coated at the same time by placing them one above another. If this is not possible on account of the design because of the number of cylinders, for example 12 cylinders, at least the connecting rods of one row of cylinders i.e., 6 for a V 12 engine, are coated lying one above another. The result of this procedure is that connecting rods of the same quality are installed in one internal combustion engine.
During thermal spraying, a gas stream is particularly advantageously passed through the connecting rod eye, in particular when a plurality of connecting rods are coated lying one above another. Air which is conditioned and purified is particularly suitable as gas stream. Preferably, the air stream should be virtually free of grease and moisture and, whenever possible, it should have a temperature within a given range, i.e. about 20xc2x0 C. The air stream advantageously has a flow velocity of about 3 m/s to 15 m/s, and in particular about 5 m/s to 8 m/s. Any overspray occurring during spraying will be blown away by the gas stream.
The bearing material is preferably sprayed using a rotating spray diffuser which is introduced into the connecting rod eye or the stacked connecting rod eyes, and which starts rotating above the connecting rod eye. Using such a spray diffuser, an especially even coating is achieved in the connecting rod eye. During coating of a connecting rod eye according to the invention, the spray diffuser is inserted into and is passed through the eye with an advancing rate of preferably about 0.5 mm/s to 20 mm/s, in particular about 2 mm/s to 8 mm/s.
During the spray application, a plurality of layers of the bearing material are preferably applied to the surface of the connecting rod eye. In particular 4 to 30 layers are preferably formed in the connecting rod eye. In the process, the layers are advantageously applied in different directions which, in turn, serves to improve the layer quality. This is achieved in the connecting rod eye by the fact that the spray diffuser coats the connecting rod eye while it is being inserted and while it is being retracted, and a rotating spray diffuser preferably maintains the same direction of rotation.
According to one aspect of the invention, the connecting rods are coated with the bearing material in series production. In this case, it is advantageous if at least some individual connecting rods from the series are measured. In this case, the average peak-to-valley height Ra, and/or the characteristics of the bearing material itself, for example the evenness of the distribution of the bearing material when a mixture is used, are measured. Such measurements of the connecting rods are preferably carried out in a nondestructive manner.